Widely practiced were processes for preparing a polyolefin resin foam wherein a volatile blowing agent is added to a melt of polyolefin resin such as styrene resin and the mixture is extruded to a low-pressure zone.
In such processes, dichlorodifluoromethane (CFC-12), dichlorotetrafluoroethane (CFC-114) and the like have been heretofore mainly used as the volatile blowing agent.
However, in recent years, it was suggested that certain kinds of flons, when released into the atmosphere, would deplete the stratospheric ozone layer and would cause global warming due to greenhouse effect, thereby inflicting a serious adverse influence on the ecosystem including humans. In this situation, an international agreement calls for a restriction of use of flons with a high ozone-depleting risk. Said CFC-12 and CFC-114 are among the flons to be controlled for restriction. From this viewpoint as well, there is a need for development of novel blowing agents which are free or substantially free from the ozone-depleting and global warming problems.
Of late, chlorodifluoromethane and 1-chloro-1,1-difluoroethane were proposed as flons which would exert little influence on the ozone layer, and they are now coming into use. Yet they contain chlorine in the molecule and are still threatening us with ozone depletion.
Further, the use of chlorine-free fluorohydrocarbon free from ozone-depleting risks was suggested in Japanese Unexamined Patent Publication No.239251/1993 which proposed the use of 1,1,1,3,3-pentafluoropropane as a blowing agent for the production of urethane foams.
Since 1,1,1,3,3-pentafluoropropane (HFC-245fa) is a nonflammable compound having a boiling point of 15.degree. C. and is a hydrogen-containing fluorohydrocarbon, the compound is considered to entail no or little ozone-depleting or global warming risks. Moreover, HFC-245fa has a boiling point close to that of trichlorofluoromethane (CFC-11) conventionally used as a blowing agent for urethane foams and it is nonflammable. Therefore, HFC-245fa is attracting attention as a promising candidate for blowing agents for urethane which substitute for CFC-11.
However, HFC-245fa exhibits a little high boiling point and a low solubility in polyolefin resins, when used as a volatile blowing agent for the production of foams of polyolefin resins such as styrene resins. Consequently a desirable foam can not be obtained because a gas is released in the die during the extrusion foaming under conventional conditions owing to the low solubility of HFC 245fa.
In this situation, there is a demand for the development of techniques for controlling the boiling point and solubility of HFC-245fa in order to effectively use CFC-245fa as a substitute for CFC-12, CFC-114 or the like.